VIBRATIONS & ELEMENTS OF AEROELASTICITY TWO MARK QUESTION AND ANSWER

VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

1. What What is simpl simplee harmon harmonic ic motio motion? n? The motion of a body to and fro about a fixed point is called simple harmonic motion. The motion is periodic and its acceleration is always directed towards the mean position and is proportional to its distance from mean position. 2. Explai Explain n the term term natura naturall frequen frequency? cy? When no external force acts on the system after giving it an initial displacement the body vibrates. These vibrations are called free vibrations and their frequency as natural frequency as natural frequency. it it is expressed in rad!sec or "ert#. $. %efine %efine the term term reson resonanc ance? e? When the frequency of external excitation is equal to the natural frequency of a vibrating body the amplitude of vibration becomes excessively large. This concept is &nown as resonance. '. Explai Explain n free and and forced forced vibr vibrati ation? on? (fter disturbing disturbing the system system the the external external excitation is removed removed then the system vibrates on its own. This type of vibration is &nown as free vibration. )imple pendulum is one of the examples. The vibration vibration which is under the influence of external force is called forced vibration. *achine tools electric bells are the suitable examples. +. %efine %efine damped damped and undamped undamped vibration vibration?? ,f the vibratory system has has a damper the motion motion of the system system will be opposed by it and the energy of the system will be dissipated in friction this type of vibration is called damped vibration. -n the contrary the system having no damper is &nown as un damped damped vibration.

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VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

. Explain Explain vibra vibration tion measuring measuring instrumen instruments? ts? The instruments which are used to measure the displacement  velocity or acceleration of a vibrating body are called vibration measuring instruments /. vibrometer vibrometer indicate indicatess 2 percent percent error error in measurin measuring g and its its natura naturall frequen frequency cy is + h#. ,f the lowest frequency that can be measured is '0 h# find the value of damping factor3 ?  45  45Ѡ!Ѡn5'0!+356  #!b51.02 #!b5 r 2!square root of 718r 232 9 2r32: 1.023;25 6;'!18'3;2 913;2  50.$+. 6. %efine %efine semi8d semi8defi efinit nitee system system?? The system having one of their natural frequencies equal to #ero are &nown as semi8 definite systems. s method? This is trial trial and error error method used used to find find the natural frequency and mode shape of multimass lumped parameter system. This can be applied to both free and forced vibrations. this method can be used for the analysis of damped undamped semidefinite systems with fixed ends having linear and angular motions. 12.Explain critical speed speed of a rotating shaft? ,t is well &nown &nown fact that rotating rotating shafts at certain speeds become dynamically unstable and large vibrations are li&ely to develop. This  phenomenon is due to resonance effects and a simple example example will show that the critical speed for a shaft is that speed at which the number of revolations  per second of the shaft is equal to the the frequency of its natural vibration. vibration. 1$. %efine self8excited vibration? We always assumed that force producing vibration is independent of the vibratory motion. ,n which a steady forced vibration vibration is sustained by forces created by the vibratory vibratory motion itself and disappearing disappearing when the motion stops .such vibration are called self excited or self induced vibration. 1'. Explain orthogonality principle? or a system with three8degree of freedom the orthogonality principle may be written as m1(1(2 9 m2@1@2 9m$A1A250 m1(2($ 9 m2@2@$ 9m$A2A$50 m1(1($ 9 m2@1@$ 9m$A1A$50 Where m1 m2 m$ are masses. (1 (2 ($ @1 @2 @$ A1 A2 A$ are the amplitude of vibration of the system. We will ma&e use of the equation 3

VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

in matrix iteration method to find the natural frequencies and mode shapes of the system.

1+. %efine matrix iteration method? With the help of this method the natural frequencies and corresponding mode shapes are determined. Bse of influence coefficients is made in the analysis. 1. Explain %un&erley>s method? This method is used to find the natural frequency of transverse viberations. The load of the system is uniformly distributed. %un&erley>s equation can be written as 1!Ѡ;251!Ѡ1;291!Ѡ2;29CC1! Ѡs;2 Where Ѡ5natural frequency of transverse vibration of shaft for many point loads. $5nat atur ural al freq freque uenc ncy y Ѡ1Ѡ2Ѡ$5n Ѡs5natural

of indi indivi vidu dual al poin pointt loa loads ds..

frequency of transverse vibration because of the weight of shaft.

1/. %efine %>(lembert>s principle? 8ma 5 0

These equation can be considered equilibrium equation provided that and

are treated as a force and a moment. This

fictitious())B*E%3 forceor moment3 is &nows as the inertia forceor 4

VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

inertia moment3 and the artificial state of equilibrium implied by above equation is &nown as dynamic equilibrium . This principle is called %>(lembert>s principle. The application of the principle to to the system shown in fig below yields the equation of motionD

or

16.%efine ewton>s law of motion? The equation of motion is =ust another another form of ewton>s law of motion F5ma total force in the same direction as motion3. Equation of motion of motion for many systems are conveniently determined by ewton>s law of motion. 16.%efine energy method? or a conservative system the total energy of the system is unchanged at all time. ,f the total energy of the system is expressed as potential and &inetic energy  then the followed is true D G.E. 9 H.E.5 constant or

G.E. 9 H.E.3 5 0

 Where the G.E. G.E. 5 &inetic energy energy H.E.5 potential energy.  The resulting equation is the equation of the motion m otion of the system under the consideration. This is  then  the Energy E nergy method.

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VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

1s *ethod? ,f the given given system is a conservative conservative one  the total &inetic energy energy of the the system is #ero at the maximum displacement but is a maximum at the static equilibrium point  for the total potential energy of the system  on the other hand  the reverse is true. "ence  

G.E.3*(I5H.E.3*(I5 Total energy of the system

This is &nown as 4ayleigh>s method. The resulting equation will readily yield the natural frequency of the system. 20.Explain the )eismic instruments. )eismic instruments are essentially vibratory systems consisting of the support or the base and the the mass with spring attached. attached. The support or the  base is attached to the body whose motion motion is to be measured. The relative motion between the mass and the base recorded r ecorded by a rotating drum or some other devices inside the instrument will indicate the motion of the body. 21.%efine vibrometer or low frequency transducer? or measuring the displacement of a machine machine part a vibrometer vibrometer should  be used  whose natural frequency frequency is low compared to the frequency of the vibration to be measured.so vibrometer is &nown as low frequency f requency transducer. 22.%efine accelerometer or high frequency transducer? (n accelerometer is used to measured acceleration because hits natural frequency is high compare to that of the vibration to be measured. )o accelerometer is &nown as high frequency transducer.

2$.%efine two degree of freedom system? )ystems that require two independent coordinates to specify their  position are called Two Two degree of freedom system.

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VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

2'.%efine normal modes of vibration or principle mode of vibration? When the masses of the system system are oscillating in such such a manner that that they reach maximum displacements simultaneously and pass their equilibrium  points simultaneously simultaneously or moving parts of the system system are oscillating in phase in one frequency such a state of the is called normal modes of vibration or  principle mode of vibration. vibration.

2+.%efine principle coordinates? ,t is also find the particular set of coordinate such that each equation of the motion contains only one un&nown quantity. Then the equation of motion solve independently of each other. )uch particular set of coordinate is called principle coordinates. %efine coordinate coupling? The displacement of one mass will will be felt experienced3 experienced3 by another another mass in the same system since they are ar e coupled together. There are two types of couplingD the static coupling due to static displacements and dynamic coupling due to inertia force. 2. %efine semi8definite system? -ne of of the roots of the frequency frequency equation equation of a vibrating system is equal to #eroJ this indicates that one of the natural frequency of the system is equal to #ero. )uch systems are &nown as semi8definite system. 2/.%efine influence coefficients? (n influence coefficient denoted by K 12 is defined as the static deflection of the system at position 1 due to unit force applied at position 2 when the force is the only force acting. The influence coefficient is therefore a convenient method to &eep account of all the induced deflections due to various applied forces and set the differential equation of the motion for the system

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VIBRATIONS & ELEMENTS OF AEROELASTICITYTWO MARK QUESTION AND ANSWER

,t can be shown that the following expression is true. Ki = 5 K =i %efine vibration of the continuous media or system? *echanical system that have their masses and elasticity distributed continuously throughout the length such as cable rods beams  plates etc. rather than LlumpedM LlumpedM together in concentrated concentrated masses by spring  belong to this class of of vibration of the continuous continuous media or system. E.g. cantilever beam 26. %efine flutter 3? ( dynamic instability occurring in an aircraft in flight at a speed is called called the flutter speed. Where the elasticity of the structure plays an essential part in the instability. 2